1. Field of the Invention
The present invention relates to a process for the surface treatment of antimony oxide pigments. 2. Description of the Prior Art
Antimony oxide has long been known for its value as a flame retardant and anti-oxidant additive for paints and plastics. However, the use of antimony oxide has several disadvantages. Variations in particle size and purity have been found to produce undesirable fluctuations in opacity from batch to batch and while the relatively high index of refraction of antimony oxide may not be detrimental in the preparation of flame retardant paints, it has often proved inappropriate for those applications where it is desired to produce a substantially transparent plastic material. Moreover, the tendency for antimony oxide to impart opacity as well as different degrees of tinting strength to plastic materials is especially undesirable when colors are to be used in the plastic. Thus, surface treatment of the antimony oxide is desirable to overcome these disadvantages.
Efforts to overcome these disadvantages include the use of phosphorus compounds and while these have been effective in producing relatively translucent plastics, the phosphates are not only expensive but may have deleterious effects on the stability of the plastic. In general, the prior art processes have each possessed the disadvantage of relatively high temperature operations to effect calcining, for example, and in other treatments. In U.S. Pat. No. 3,740,245 issued to White et al., a flame retardant additive for plastics is prepared by reacting antimony oxide with a substrate comprising hydrous silica, hydrous alkali or alkaline earth metal silicate in a weight ratio in the range of from 1:4 to 1:1 on an antimony oxide to substrate basis. The reaction is carried out by calcining an admixture of said compounds at a temperature within the range of from 400.degree. to 550.degree. C in an oxygen containing atmosphere.
U.S. Pat. No. 3,560,441 issued to Schwarcz discloses another means for insuring improved translucency and uniform color in plastics wherein antimony oxide is mixed with a compound of boron and fused to form a boron-glass by heating the admixture to a temperature of from about 650.degree. to 1100.degree. C for a period of time sufficient to melt the admixture followed by rapid cooling and grinding to a preferred particle size. However, this process also has the disadvantages attending a relatively high temperature operation requisite to the formation of a glass melt followed by quenching to produce a glass frit and subsequent grinding to provide a prescribed particle size.
As previously mentioned, antimony oxide has been used as a flame retardant in paints and U.S. Pat. No. 2,882,178 issued to Dunn et al., discloses an improvement wherein the antimony oxide is coated on a substrate of particulate silica. While this material compares favorably with antimony oxide as a flame retardant additive, the antimony coated silica is relatively abrasive and hence causes excessive wear of processing equipment attended by the inclusion of metallic contaminants in the final produce thus causing marked graying in the end product.